1. Field of the Invention
The present invention relates to a photoelectric conversion element. Moreover, the present invention relates to a photoelectric conversion circuit including the photoelectric conversion element, and a display device including the photoelectric conversion circuit.
2. Description of the Related Art
Nowadays, semiconductor devices are indispensable to human life. Semiconductor elements, such as thin film transistors included in semiconductor devices are manufactured in such a manner that a semiconductor film is formed over a substrate and the semiconductor film is processed into a desired shape by a photolithography method or the like. Such a manufacturing method is used for forming a liquid crystal display device (e.g., a liquid crystal television) or the like.
An amorphous silicon film has been often used as a semiconductor film in a thin film transistor of a conventional liquid crystal television. This is because a structure of a thin film transistor formed using an amorphous silicon film is thought to be manufactured with relative ease. However, when the current circumstances of moving images (e.g., watching movies and sports in 3D) are taken into account, a thin film transistor which responds at higher speed than those using an amorphous silicon film has been developed. For example, a thin film transistor using a microcrystalline silicon film has been developed (e.g., Patent Document 1).
On the other hand, an optical touch panel has been developed. However, a current touch panel has problems that material cost is high and yield is not favorable, because a light-receiving element is generally formed over a substrate which is different from a substrate of a display panel and mounted.
Thus, attempts to form a pixel with the use of a thin film transistor using a microcrystalline silicon film and realize a photoelectric conversion element formed in the same step as the thin film transistor (e.g., Patent Documents 2 and 3) have been made. Note that a photoelectric conversion element having a dual-gate structure in which a back gate is provided has been disclosed (e.g., Patent Document 4).